4Q METALLURGY OF IRON AND.STEEL.
the walls, so that even if a patch of lining is carried away, the ioss is restored by the furnace itself by a carbon lining upon the cold plate. -
Half a century ago there were few furnaces in the world as much as 50 feet high, hut it was found that an increase to 70 feet saved fuel and increased the output. It was natural to assume; that a greater height would insure greater economies, and during the last quarter of a century there has been a race in Eastern Amer*ca to build the biggest furnace and turn out the most iron. In 1875 a big furnace was 80 feet high and made 100 tons per day. Now
there are stacks 100 feet high, making. 600 tons........It improbable that
this is the .commercial limit of size, not on account of inability to operate a larger furnace, but because in a steel works it is more convenient to have six furnaces, making 400 tons per day, [than to have four furnaces making 600 tons,, as an accident to ojie unit causes less interruption to tributary departments. It is also found that, on Lake Superior. ..ores, little is gained by increasing the height beyond 90 feet, ' "' : ' . j
SEC. lib.—Ore.—Three kinds of ore .are used in the (making of iron: (1) hematites, (2) carbonates and (3) magnetites! They never occur in a pure state, being mixed with earthy materials, but in discussing their composition it is, necessary to consider the iron
mineral by itself.. • --- • ,":- ... ".....•-•-•-——........•
(1) .-Hematite (Fe203) contains exactly 70 per cent, of iron, but in addition to -ordinary earthy impurities it carries-water- of crystallization in amounts up ,to 20 per cent. When the proportion of this water is low the ore! is called a "red" or "brown" hematite, while the hydrous varieties are called "soft" -hematites," "or "limo-nites,";;although this latter term should only be applied! to bog ores containing about 20 per cent.,: This water of crystallization can only be removed by heating the ore nearly to a red heat. Oolite is a variety of hematite'composed of small spherical, grains, each' grain being a kernel of foreign matter surrounded,";by iron, ore. When the .foreign matter is silica, as in some places in Alabama, the ore is well nigh worthless, but when it-is partly lime, as in the Minette district of Germany and Luxemburg, the ore is "self-fluxing." : If such an ore carries 40 per cent., of •irion, and sufficient lime :so .that .no .stone is needed in the furnace, it is as valuable as an;'0re.with.:50..per cent.,of.iron .and..nO'-limd It is